Normothermic tumors showed a circadian rhythm with zenith at midnight and nadir at midday with differences that could reach 3 degrees C. Abdominal tumors were 0.5 to 1.0 degrees C warmer than subcutaneous tumors. The non-uniformity of temperature that existed within normothermic tumors was exaggerated during hyperthermia. No appreciable change in temperature gradients within a tumor was obtained when tumor blood flow was doubled or reduced to one-third. Decrease or increase of blood flow during hyperthermia did not eliminate the sharp temperature gradients. A cold pulse of serum into the tumor efferent artery produced a substantial reduction of tumor blood flow but a small depression of tumor temperature and a very small change in tumor temperature gradients. The data demonstrate extensive anisotropy of blood distribution within a tumor and suggest that thermal diffusion more that convection controls heat transfer within a tumor.